Scientists think they have modeled the brain mechanisms that let monarch butterflies, year after year, migrate thousands of miles in one direction: south, to Mexico.

While researchers already knew from prior analyses that the winged wonders use two primary bits of input data to navigate – time of day and position of the sun on the horizon – they had no answer for how the butterfly's brain gathers and processes that information.

Until now, say researchers from the universities of Washington, Michigan and Massachusetts.

In a paper published in the journal Cell Reports, a team from those institutions says it has modeled the neural control mechanisms at work in the butterfly's brain.

"We wanted to understand how the monarch is processing these different types of information to yield this constant behavior - flying southwest each fall," said study co-author Eli Shlizerman, an assistant professor at the University of Washington, in a statement.

Monarchs watch the sun, but that's not enough to get them where they need to go. They also need the time of day. Monarchs, just like people, have a kind of internal clock that keeps track of the daily go-round. Unlike people, though, their clock is centered in the antennae (an appendage humans have so far disdained).

Shlizerman and his colleagues recorded neural signals coming from monarch antenna nerves, to keep track of the clock information, as well as information coming from the insect's eyes, to account for sun position.

Then they used that data to create a circuit model to show the neural control mechanisms – for the "clock" and "sun position" – that help the butterfly point itself southwest.

The scientists' model even accounts for how the butterflies get back on course if they stray.

And what about when it's time to head back north? The team's model suggests that the neural mechanisms just reverse direction.

"And when that happens, their compass points northeast instead of southwest," said Shlizerman. "It's a simple, robust system to explain how these butterflies - generation after generation - make this remarkable migration."

The scientists say more study will be needed to fully ensure their model matches up with the butterfly's brain, body structures, and behavior. For now, though, they say the model they've created appears to be consistent with behavior seen in the legendary migratory creatures.

Between September and November, North American monarch butterflies make a staggering fall migration that may take as many as 3,000 miles to complete.
In appreciation of that arduous journey, let's enjoy a few pictures of these winged beauties and learn a bit about their amazing trip.

During the summer, it's breeding season, and ordinary monarchs will live about 2-5 weeks, during which they'll mate and lay eggs. Those eggs will become the next generation, each successive generation flying a bit further north than the last.
All told, there are four generations of monarchs per year.
It's the fourth, and last, generation that's special. Born in the fall, they are hard-wired to migrate. As such, they don't become reproductive right away. (They'll do that later, on the return flight north, having lived for as long as nine months!)
The migration kicks in once temperatures drop. Monarchs don't handle cold weather very well. Too deep a chill affects their ability to fly. So when cooler air and shorter days come in late summer and early fall, it's time for them to pack their bags and head south.

Generally, monarchs west of the Rocky Mountains will overwinter in central and southern California, while those east of them will head for Mexico's Michoacan Mountains.
They use the same overwinter sites year after year. Sometimes even the same tress, notes

, a site devoted to all things monarch butterfly.
The precision of their navigation is still a bit of a mystery. It's thought they find their way using the sun and a kind of solar compass in the brain. But last summer

As they continue southward, only flying during the day, the butterflies will make pit stops to rest and to nectar. Astonishingly, they gain weight along the way, even though the journey is unspeakably long and draining for something so small and delicate.

Along the way, adult monarchs will dine on the nectar of a great many plants. Wild carrot, lilac, and alfalfa to name just a few.
And, as we can see, when it's time to crash, they don't mind living in close quarters.

The eastern U.S. monarchs that head to Mexico will overwhelmingly make their way to the Monarch Butterfly Biosphere Reserve, a World Heritage site that is home to millions of monarchs from October through March.
These monarchs have reached their destination and are just kicking back in Mexico.

Once February and March arrive, spring is near. And -- guess what? -- it's time for these well-traveled monarchs to gear up and make a return flight north, as temperatures there warm up.
Only now do they become reproductive. As they head back north, they will lay eggs on milkweed to seed a new generation.
Their descendants will return to the same overwintering sites from which they returned. Ain't nature grand?